The present invention relates to the customization of electronic devices using radiant energy to configure fuses. More particularly, the present invention relates to a compact arrangement of fuses and active circuitry on an integrated circuit.
Fuses and antifuses have been used in the manufacture and repair of integrated circuits for some time. The word fuse will be used throughout this specification to mean fuse, antifuse, or both. The uses of fuses have been in: (1) the repair of circuits through the selective addition and deletion (or substitution) of circuitry to repair bad portions of circuitry; (2) the marking of units for identification; (3) the customization of circuits by altering their structure, paths, or electrical characteristics. It has been common for a fuse to have an open area above the fuse to allow for the application of a radiant energy beam to the fuse from above the integrated circuit. It has also been common for the area around the fuse to be free of active circuitry (transistors, resistors, signal lines, junctions, etc.) for a distance larger than the minimum feature dimensions of the process used. The reason for this practice is that the spot size of the radiant energy beam has been larger than the feature size of the fuse, and the nearby circuitry could not be subjected to the heat of that radiant energy pulse, for fear that it would be damaged. Additionally, the area below the fuse has been kept clear of active circuitry (transistors, resistors, signal lines, junctions, etc.) for fear that the radiant energy used to configure the fuse would damage this active circuitry. Thus, the use of radiant energy configurable fuses for custom integrated circuits will increase the overall chip area of the integrated circuit since active circuitry could not be placed underneath or near these fuses.
There is therefore a need for increasing the density of the integrated circuit that utilizes radiant energy to configure fuses.
Accordingly, it is a feature of the present invention to increase the density of the integrated circuit by allowing active circuitry below the beam area of the radiant energy used to configure the fuses.
It is another feature of the invention to enable the use of higher energy radiant energy beams for improved fusing by protecting the active circuitry from the radiant energy used to configure the fuses.
It is another feature of this invention to enable the fuses to be made with materials that require higher radiant energy to configure them by protecting the active circuitry from the radiant energy used to configure the fuses.
Another feature of the present invention is to allow the area around the fuse to be cleaned up with one or more additional radiant energy pulses, with the reflected energy from the protective layer providing a more complete removal of the fuse material.
It is yet another feature of the invention to provide protection of the underlying active circuitry from the energy of the clean-up pulse since the fuse is no longer present to absorb the radiant energy.
These and other related features are achieved through the use of the novel protection method and integrated circuit structure disclosed. In accordance with an aspect of the present invention, an integrated circuit where one desires the most compact arrangement of fuses and active circuitry, an insulating layer is deposited over active circuitry which includes the associated interconnect layers. The insulating layer may be planarized or not. Optional vias may be etched at this time or later to a lower conducting surface for interconnection. A protective layer made with a substantially non-transmissive material, preferably a reflective material (aluminum, titanium or the like), is disposed above the lower layers of the integrated circuit containing active circuitry which includes interconnect layers of any desired number. This protective layer, which may also be used as a conductive interconnect layer, is patterned below the areas that will later contain fuses. Above this protective layer another insulating layer is deposited. A fuse layer may be metal or another conductive film such as polysilicon, amorphous polysilicon, silicide, or other suitable material for a fuse is then deposited. This conductive layer is patterned to provide the desired fuses as required, with some or all of the fuses aligned with the protective layer deposited underneath. The protective layer is patterned such that the area of the protective layer underneath the fuses will absorb and/or deflect much of the radiant energy that does not directly impinge upon the fuses.
In accordance with another aspect of the invention, an integrated circuit comprises active circuitry and a first insulating layer overlaying the active circuitry. A fuse layer is disposed above the first insulating layer, and includes at least one fuse. The fuse is radiant-energy configurable, and has a location such that the beam area of the radiant energy used to configure the fuse overlaps the active circuitry. A first protective layer is underneath the fuse, and is sufficiently large to shield the active circuitry from the radiant energy not directly impinging upon the fuse. A second insulating layer is disposed between the protective layer and the fuse.
In accordance with another aspect of this invention, a method for protecting active circuitry on an integrated circuit from radiant energy used to configure the integrated circuit comprises the steps of providing a fuse layer having at least one fuse and providing a protective layer underneath the fuse. The fuse has a location such that the beam area of the radiant energy used to configure the fuse overlaps the active circuitry. The method further includes the step of configuring at least one of the fuses using radiant energy.
In one embodiment the protective layer is used as a conductor, such as a power, a ground conductor, or other lines that preferably are not minimum dimension lines, further increasing the conductor packing efficiency and usefulness of the protective layer.
In another embodiment the protective layer may be made of more than one material to increase its protective capability or to perform another useful purpose such as a capacitor structure. For instance, the protective layer may comprise sandwiches of various materials designed to better reflect or absorb the radiant energy without damage.
In yet another embodiment the protective plate may be formed under the fuse in such a manner that it is tilted from the horizontal on one or both sides of the fuse, and it may include more than one piece. In this configuration, the radiant energy that misses the fuse off to one or both sides may be reflected back to the underside of the fuse. It also allows more of the incident radiant energy to be directed to the fuse than would normally be captured by the width of the fuse itself. This configuration further allows the fuse to be heated from both the top and the bottom to more completely configure the fuse.
It has been common for interconnect layers to be formed with insulating layers between them. In another embodiment, the protective layer may be formed in interconnect layers other than that immediately below the fuse. This allows two or more insulating layers to be present between the protective layer and the fuse. The combination of these insulating layers would be thicker and stronger than a single insulating layer. Thus, the combined insulating layers would be more resistant to rupturing and would enable the protective layer to absorb more radiant energy.
In another embodiment the protective layer may be formed with openings such as small holes or slits to further strengthen the insulating layer due to the additional attachment points to the layers underlying the protective layer. If these openings have at least one dimension which is less than the primary wavelength of the radiant energy, diffraction will significantly diffuse radiant energy which reaches the active circuitry below. The openings may also be placed generally under the fuses if the dimensions of the openings are small relative to the fuse width plus the radiant energy wavelength.
In another embodiment the fuse layer may have an anti-reflective coating where the fuses are formed. This enables the fuse to better absorb the radiant energy used to configure it.